1. Field of the Invention
The present invention relates to a laser processing apparatus suitable for formation of a laser processed groove by applying a laser beam along a street formed on the front side of a workpiece such as a semiconductor wafer.
2. Description of the Related Art
In a semiconductor device fabrication process, a multilayer film composed of an insulating film and a functional film is formed on the front side of a semiconductor substrate such as a silicon substrate, and a plurality of devices such as ICs and LSIs are formed like a matrix from this multilayer film, thus obtaining a semiconductor wafer having the plural devices. The plural devices are partitioned by a plurality of division lines called streets formed on the front side of the semiconductor wafer. The semiconductor wafer is divided along these streets to obtain the individual devices.
Division of the semiconductor wafer along the streets is usually performed by using a cutting apparatus called a dicing saw. This cutting apparatus includes a chuck table for holding the semiconductor wafer as a workpiece, cutting means for cutting the semiconductor wafer held on the chuck table, and moving means for relatively moving the chuck table and the cutting means. The cutting means includes a spindle adapted to be rotated at high speeds and a cutting blade mounted on the spindle. The cutting blade is composed of a disk-shaped base and an annular cutting edge mounted on a side surface of the base along the outer circumference thereof. The cutting edge is formed by bonding diamond abrasive grains having a grain size of about 3 μm by electroforming so that the thickness of the cutting edge becomes 20 to 30 μm, for example.
In recent years, a semiconductor wafer intended to improve the processing performance of devices such as ICs and LSIs has been put into practical use. This semiconductor wafer is composed of a semiconductor substrate such as a silicon substrate and a multilayer film formed on the front side of the semiconductor substrate. The multilayer film is composed of a low-permittivity insulator film (Low-k film) and a functional film formed on the Low-k film, the functional film forming a plurality of circuits. Thus, the semiconductor devices are formed from the multilayer film. Examples of the Low-k film includes an inorganic film of SiOF, BSG (SiOB), etc. and an organic film such as a polymer film of polyimide, parylene, etc. Further, there is also in practical use a semiconductor wafer having a configuration such that a plurality of metal patterns called test element group (TEG) are partially provided on the streets of the semiconductor wafer to test the function of the circuits through the metal patterns before dividing the semiconductor wafer.
The Low-k film or the test element group (TEG) mentioned above is different in material from the semiconductor substrate, so that it is difficult to cut the semiconductor substrate together with the Low-k film or the test element group (TEG) by using the cutting blade. That is, the Low-k film is very brittle like mica. Accordingly, when the semiconductor wafer having the Low-k film is cut along the streets by using the cutting blade, there arises a problem such that the Low-k film may be separated and this separation may reach the devices to cause fatal damage to the devices. Further, the test element group (TEG) is formed of metal. Accordingly, when the semiconductor wafer having the test element group (TEG) is cut along the streets, burrs may be produced. To solve these problems, there has been proposed a processing method including the steps of applying a pulsed laser beam along the streets on the semiconductor wafer to thereby remove the Low-k film forming the streets or the test element group (TEG) provided on the streets and next cutting the area where the Low-k film or the test element group (TEG) is now absent by using the cutting blade (see Japanese Patent Laid-Open No. 2005-142398, for example).
However, in the case of removing the Low-k film or the test element group (TEG) by applying a pulsed laser beam along the streets of the semiconductor wafer as in the processing method disclosed in Japanese Patent Laid-Open No. 2005-142398, it is necessary to form a laser processed groove having a width larger than the thickness of the cutting blade along each street. Accordingly, in the case that the focused spot diameter of the laser beam is about 10 μm, a laser beam applying step of applying the laser beam along each street must be performed plural times as shifting the focused spot along the width of each street, causing a reduction in productivity.
To solve this problem, there has been proposed a laser processing apparatus as described in Japanese Patent Laid-Open No. 2011-156551. This laser processing apparatus has a configuration such that a laser beam is branched into a plurality of laser beams to thereby form a plurality of focused spots (focal points), which are arranged along the width of each street to form a wide laser processed groove at a stroke, so that the Low-k film or the test element group (TEG) can be efficiently removed.